1. Field of the Invention
This invention relates to sequential resonant tunneling (SRT) photodetector and emitter devices based on III-nitride multiple-quantum-well (MQW) structures; to methods of their fabrication; and to methods of their use, where III-nitride refers to compound alloys of nitride and group III elements including aluminum, gallium, and indium.
2. Description of the Related Art
III-nitride-based photodetectors are used in space-to-earth and space-to-space communication, missile plume detection, detection of biological organisms and bacteria, combustion sensing and control for aircraft engines, optical storage, air quality monitoring, cancer diagnosis, and personal ultraviolet exposure dosimetry. Alloys of III-nitride are becoming the semiconductor of choice for photodetectors and light emitters in the wavelength range from yellow to ultraviolet due to their direct and wide band gaps. Because of their thermal stability and radiation hardness, these materials are remarkably tolerant in aggressive environments. By designing the semiconductors to have different mole fractions of group III elements in the nitride compounds, the cutoff wavelength of III-nitride-based detectors is adjustable in a wide wavelength range from 630 nm to 200 nm. This approach makes selective spectral detection realizable. As the result of the rapid progress of growth techniques, such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), ultraviolet photodetectors based on GaN bulk material have even been commercialized. However, quantum efficiency and response speed of bulk-based photodetectors are limited by their low absorption efficiency and low carrier mobilities.
U.S. Pat. No. 6,649,943 to Shibata et al, for “Group III Nitride Compound Semiconductor Light Emitting Element”, discloses a group III nitride compound semiconductor light-emitting element formed of group III nitride semiconductor layers, including a multi-layer containing light-emitting layers; a p-type semiconductor layer; and an n-type semiconductor layer, wherein the multi-layer includes a multiple quantum barrier-well layer containing quantum-barrier-formation barrier layers formed from a group III nitride semiconductor and quantum-barrier well layers formed from a group III nitride compound semiconductor, the barrier layers and the well layers being laminated alternately and cyclically, and a plurality of low-energy-band-gap layers which emit light of different wavelengths, with the multiple quantum barrier well layer being provided between the low-energy-band-gap layers.
The devices of Shibata et al are electrically pumped light emitters, which are used as UV and visible light sources. The active layers of these devices are the low-energy-band-gap layers adjacent to the multiple quantum barrier-well layer. The devices operate under a forward bias that creates carrier injection to the low-energy-band-gap layers. The injected carriers recombine in the low-energy-band-gap layers and generate UV and visible light. These devices use inter-band carrier recombination in the low-energy-band-gap layers. Electrons from the n-type layer and holes from the p-type layer are injected to the low-energy-band-gap layers and recombine there to produce UV and visible light. These devices do not rely on the phenomenon of sequential resonant tunneling. The measurable output signal of these devices is the optical power of the emitted light. The quantum efficiency of these devices is evaluated by the fraction of injected electrons and holes that recombine in the low-energy band-gap layers and successfully generate UV and visible photons.